(1) Field of the Invention
The present invention relates to a lighting device for a discharge lamp capable of lighting it at a low DC voltage.
(2) Description of the Related Art
A conventional power unit for a discharge lamp, for example, as disclosed in Japanese Patent laid-open publication No. 2001-128461, pages 7 and 8, FIGS. 1 to 3, shares a switching element constituting a bridge circuit and a chopper circuit for lighting a discharge lamp.
However, the aforementioned disclosed art, when a DC power source at a low voltage such as a battery is used, to obtain the voltage for lighting the lamp during the stable lighting period of the lamp, the duty ratio for turning on or off the switching element for the chopper must be changed, and a difference is caused in consumption of the lamp electrode, thus a problem arises that the life span of the lamp is reduced.